1. Field of the Invention
The present invention relates to a method and an apparatus for mapping a baseband signal into a beamspace, and more particularly, to a method and an apparatus for mapping a baseband signal into a beamspace (specific antenna port) in a communication system.
2. Description of Related Art
In recent years, various communication technologies have adopted a multiple inputs multiple outputs (MIMO) technique. The MIMO technique is an essential method for increasing a data transmission rate and may expect an effect of maximizing frequency efficiency. As the representative example, there are IEEE 802.16 and IEEE 802.20 of a portable Internet system and a standard of Wibro system and a cellular communication system called long term evolution (LTE) of 3th generation partnership project (3GPP) has also adopted the MIMO technique.
The transmission performance in the MIMO communication system is generally increased in proportion to the number of antennas. Therefore, to maximize the MIMO performance, the number of antennas needs to be increased and thus the number of radio frequency (RF) chains needs to be increased. However, if the number of antennas is increased, complexity of the system is increased and a size of the system is increased. Therefore, there is a disadvantage in that the number of antennas may not be increased blindly. To overcome the restrictions, recent researches for achieving MIMO performance using a single RF chain have been conducted. As the representative example, there is a single RF MIMO technology using an electrical steering parasitic array radiator antenna (ESPAR) or a load modulation antenna.
Compared to the existing technology, the technology takes a slight different form in terms of an antenna/RF and a baseband. Referring to the antenna/RF, the existing MIMO technology uses a plurality of active antenna elements to configure an antenna but the single RF MIMO technology uses one active antenna element or a small number of active antenna elements and a plurality of parasitic antenna elements to configure an antenna. The structure has an advantage of expanding the number of antennas through the plurality of parasitic antenna elements and disposing a distance between the parasitic antenna elements to be narrower than the general active antennas to reduce the size of the antenna. Further, the single RF chain is used and therefore the RF chain is not complicated, such that the size of the RF chain may be implemented to be small. Further, when the distance between the antennas is not a great problem and the complexity of the RF chain is a main issue, the antennas may be disposed by increasing the distance therebetween using the load modulation antenna and the single RF chain.
Referring to the baseband, the existing MIMO technology uses the plurality of active antenna elements, and therefore I/Q signals for the baseband signals are mapped into each antenna and the performance does not depend greatly on into which of the antennas each signal is mapped
In contrast, the single RF MIMO technology has only one RF chain and therefore does not use a scheme of mapping the baseband signals into each antenna but use a scheme of mapping all antenna beam patterns generated by one active antenna element and a small number of active antenna elements and the plurality of parasitic antenna elements into a plurality of decomposed orthogonal beams. At this point, the beam into which the baseband signal is mapped has different shapes and therefore there is a difference in the transmission performance as the existing technology. That is, there are various methods for mapping the baseband signal into which of the beams and a specific method may not properly secure the transmission performance. Therefore, there is a need for a method for mapping a baseband signal into a beam while securing proper transmission performance when a single RF MIMO is used.